1. Field of the Invention
The present invention relates to photocurable compositions, to flexographic printing plates derived from said compositions and to flexographic printing relief forms prepared from said plates. More in particular, the present invention relates to compositions showing an improved combination of curability and ozone resistance.
2. Background
Photocurable printing plates are known for use in making flexographic printing forms. The printing surface is produced by exposing a photocurable layer image-wise to actinic radiation and subsequently removing the unexposed, non-photocured areas of the printing plate. Examples are found in the following patents: DE C-2215090, U.S. Pat. No. 4,266,005, U.S. Pat. No. 4,320,188, U.S. Pat. No. 4,126,466, U.S. Pat. No. 4,430,417, U.S. Pat. No. 4,460,675 and U.S. Pat. No. 5,213,948.
Such photocurable printing plates usually comprise a support, an optional adhesive layer or other underlayer, one or more photocurable layers, an optional elastomeric intermediate layer and a cover layer. A preferred method for making such multilayer photocurable printing plates is by a process in which a previously extruded photocurable composition is fed into the nip of a calendar and is calendered between a support layer and a cover layer, thus forming a photocurable layer between them. EP B-0084851 disclosed a preparation method for a multilayer photocurable printing plate, having an added elastomeric layer between the cover layer and the photocurable layer.
The photocurable layers contain polymeric binders, photocurable monomers, photo-initiators, and added auxiliaries such as plasticizers, fillers, stabilizers etc. The polymeric binders are usually thermoplastic elastomeric block copolymers, as disclosed in e.g. U.S. Pat. No. 6,531,263 and DE-C-2,215,090. These are generally block copolymers of the general formulae A-B-A or (AB)n or (AB)nX, comprising thermoplastic blocks A and elastomeric blocks B, particularly linear and radial block copolymers with poly(monovinyl aromatic hydrocarbon) end blocks.
Unhydrogenated styrene diene block copolymers, both the SIS and SBS types, can be used to make UV cured, flexographic printing plates as taught in U.S. Pat. No. 4,427,759. Although they are widely used and give excellent performance, one limitation is that the plates made with the unsaturated polymers have limited resistance to degradation by ozone. This is becoming more of a problem as UV cured inks become more widely used since ozone is generated from oxygen during the UV irradiation used to cure the ink.
Now an improved photocurable composition based on selectively hydrogenated block copolymers having both hydrogenated conjugated diene blocks and unsaturated conjugated diene blocks has been found. This improved composition leads to high gel contents, or degree of cure, after photocure. The higher gel content is expected to result in improved resolution and dimensional stability of the flexographic printing plate. Furthermore, the selectively hydrogenated character of the block copolymer comprising the photocurable composition is expected to be resistant to ozone generated during photocure of the flexographic plate itself and of UV curable inks.